The use of tabular silver halide grains in photographic emulsions, and the preparation thereof, have been widely known in the art. Tabular grains provide many advantages which have been well documented in the art. Generally, tabular grains are flat, silver halide grains that are prepared by employing long ripening times or a controlled salt addition such as provided by the balanced double jet (BDJ) method. The conventional tabular grain preparation procedure involves the steps of:
a) nucleation, wherein nuclei are formed upon which the grain will eventually grow; PA1 b) an optional Ostwald ripening to dissolve the smaller nuclei and redeposit them on the larger, more stable nuclei; PA1 c) growth, wherein additional silver salts and halide salts are added such that the nuclei formed during nucleation are grown to the desired size; PA1 d) optional Ostwald ripening. PA1 a) adding silver nitrate at a bromide ion concentration of 0.08 to 0.25 N (pBr of 0.60 to 1.10); PA1 b) adding a silver halide solvent, i.e. ammoniacal base, after at least 2% of the total silver solution is added; PA1 c) optional halt, or Ostwald ripening, at a bromide ion concentration of 0.005 to 0.05 N (pBr of 1.30 to 2.30) PA1 d) optional neutralization; PA1 e) growth. PA1 a) forming a nucleation solution by adding aqueous soluble silver salt to a vessel containing 0.08 to 0.25 molar aqueous soluble halide salt in an aqueous dispersion medium PA1 b) adding a solution of ammoniacal base to said nucleation solution when 0.30 to 9.0% by weight of said soluble silver salt has been added; PA1 c) optionally ripening said nucleation solution for up to 60 minutes; PA1 d) adding said soluble silver salt to said nucleation solution preferably at an increasing rate to obtain growth pBr of 1.3 to 2.3; PA1 e) adding said soluble silver salt and said soluble halide salt to grow said tabular grains. PA1 a) forming a nucleation solution by adding aqueous soluble silver salt to a vessel containing an 0.08 to 0.25 molar aqueous soluble halide salt; PA1 b) adding a solution of ammoniacal base to said nucleation solution when 0.30 to 9.0% by weight of said soluble silver salt has been added; PA1 c) ripening said nucleation solution at a pBr level of no less than 0.780 and no more than 1.250; PA1 d) adding said soluble silver salt to said nucleation solution at an increasing rate to obtain growth pBr of 1.3 to 2.3; PA1 e) adding said soluble silver salt and said soluble halide salt to grow said tabular grain.
The increased demand for tabular grains has placed a burden on manufacturers to decrease the time required to manufacture tabular grains. This demand increases as the size of the tabular grain increases since a larger period of time is required during growth to add the precursor salts of silver and halide. Simply increasing the addition rate of the precursor salts, as advanced in U.S. Pat. No. 4,945,037, causes the formation of new nuclei during the growth phase which is undesirable. The nuclei formed during growth do not reach the same size as the grains which are grown upon the nuclei formed during the nucleation step. This undesirable growth of the newer nuclei requires a second Ostwald ripening after grain growth is completed to decrease the number of smaller grains. This extra processing step is undesirable due to the extra time involved and the inherent decrease in productivity.
Silver solvents such as ammonia have been taught in U.S. Pat. Nos. 4,727,886 and 4,801,522 to assist in the dissolution of small particles during Ostwald ripening, and to decrease the formation of new nuclei during grain growth. Specific process steps comprise
Furthermore, the addition of silver solution during the growth phase was maintained at a constant flow rate until the growth pBr was obtained. This approach works well if the rate of salt addition during growth is maintained at a relatively low rate of addition. At high levels of salt addition the salts mix to form new grain nuclei which grow as described above. If, during the growth phase, the rate of formation for new grain nuclei exceeds the rate of dissolution, multiple sizes of grains are produced. Some grains will be grown from the seeds formed during nucleation and smaller grains will be formed from the seeds formed during growth. The resulting emulsion may suffer in two ways. First, large size grains may not be obtained since there are more seeds competing for added salts. Second, the size distribution may suffer if the newer seeds do not reach the size of the particles intentionally formed during the initial nucleation phase. U.S. Pat. No. 5,028,521 further extends the growth period prior to ammonia addition wherein at least 20% of the silver is added prior to addition of the ammoniacal base. Increasing the amount of silver added prior to the addition of ammoniacal base does not correct the deficiency of the previous teachings.
The ability to form tabular grains in shorter time has been provided in U.S. Pat. No. 5,013,641 by addition of sodium hydroxide. To exploit the teachings, the initial seeding, or nucleation, must be done at a very low level of gelatin or peptizer. An additional step is then required wherein additional gelatin is added for growth. Furthermore, the nuclei formation must be accomplished with a dilute silver solution followed by growth with a concentrated silver solution. These extra process steps are highly undesirable.
There is an ongoing need in the art for a process for large tabular grain manufacture which can be accomplished in a short period of time without adversely affecting the size distribution of the grains and without increasing the number of process steps.